This application claims the priority benefit of Taiwan application serial no. 91105535, filed Mar. 22, 2002.
1. Field of Invention
The present invention relates to an automatic gain control (AGC) circuit. More particularly, the present invention relates to an automatic gain control circuit using a feedback control to modify the gain factor of a voltage gain amplifier (VGA).
2. Description of Related Art
In analogue signal processing circuits for magnetic or optical disc, the received input signals are often varied due to different pick-up devices or storage mediums. Hence, an automatic gain control circuit is frequently employed to maintain proper output signals for subsequent signal processing. And in order to maintain a proper output signal level, the gain of the automatic gain control circuit needs to be varied according to the received input signal. In other words, the gain is decreased when the input signal level is increased and the gain is increased when the input signal level is reduced.
FIG. 1 is an equivalent circuit diagram of a conventional automatic gain control circuit 100. As shown in FIG. 1, the automatic gain control circuit 100 includes a variable gain amplifier (VGA) 110, a low-pass filter 120, an amplifier 130 (having a fixed gain), a full-wave rectifier 140, a comparator 150, a charge-pump circuit 160 and a capacitor 170. Differential input signals VIA, VIB produce output signals DP, DN after passing through the amplifier circuits and the filtering circuit such as the variable gain amplifier 110, the low-pass filter 120 and the fixed-gain amplifier 130. After rectification of the output signals DP, DN by the full-wave rectifier 140, the rectified signal is passed to the comparator 150 to compare with a required magnitude VAGCDAC to produce a compare result signal Vcp. Utilizing the compare result signal Vcp, the charge-pump circuit 160 is controlled to charge or discharge the capacitor 170 and vary the potential difference across the capacitor 170 so that the gain of the variable gain amplifier 110 can be adjusted. When the amplitude of the output signals DP, DN is greater than the required magnitude VAGCDAC, the comparator 150 outputs a high level compare result signal VCP to control the current Iattack within the charge-pump circuit 160 and charge up the capacitor 170. Hence, reducing the gain of the variable gain amplifier 110. On the other hand, when the amplitude of the output signals DP, DN is smaller than the magnitude VAGCDAC, the comparator 150 outputs a low level compare result signal VCP to control the current Idecay within the charge-pump circuit and discharge the capacitor 170. Ultimately, the gain of the variable gain amplifier 110 is increased. Nevertheless, this type of automatic gain control circuit is limited by the bandwidth of the full-wave rectifier 140. When the input signals contain high frequency components, a full-wave rectifier having a bandwidth wide enough to accommodate the high frequency components of the signals is difficult to implement. In addition, the gain of a full-wave rectifier also affects precision of the entire circuit.
FIG. 2 is an equivalent circuit diagram of another conventional automatic gain control circuit. As shown in FIG. 2, the automatic gain control circuit 200 includes a variable gain amplifier 210, a low-pass filter 220, a fixed-gain amplifier 230, a pair of programmable level shifters (240, 260), a pair of comparators 250, 270, an OR-gate 280, a charge-pump circuit 290 and a capacitor 295. The principle of operation is very similar to the automatic gain control circuit shown in FIG. 1 except with an improvement over the bandwidth limitation of the full-wave rectifier 140 of FIG. 1. The full-wave rectifier 140 and the comparator 150 for generating the compare result signal VCP in FIG. 1 are replaced by the programmable level shifters 240, 260, the comparators 250, 270 and the OR-gate 280 in FIG. 2. Here, only the operation of the programmable level shifter 240 the comparator 250 is illustrated with reference to FIGS. 3 and 4. FIG. 3 is a graph showing the waveforms of the output signals DP, DN and a shifted output signal DNS after the output signal DN passes through the programmable level shifter 240 to shift by an amount of Vs. FIG. 4 is a graph showing the waveform of a first compare result signal VCOMP1 after passing the shifted output 15 signal DNS into the comparator 250 to compare with the output signal DP. When the amplitude of the output signals DP, DN is large, the comparators 250, 270 will output high level first and second compare result signals VCOMP1 and VCOMP2 respectively. According to the first and the second compare result signals VCOMP1 and VCOMP2, the OR-gate produces a compare result signal VCP to control the current Iattack of the charge-pump circuit 290 to charge the capacitor 295. Hence, reducing the gain of the variable gain amplifier 210. On the other hand, when the amplitude of the output signals DP, DN is too small, the comparators 250, 270 output low level first and second compare result signals VCOMP1 and VCOMP2. According to the first and the second compare result signals VCOMP1, and VCOMP2, the OR-gate produces a compare result signal VCP to control the current Idecay of the charge-pump circuit 290 to discharge the capacitor 295. Ultimately, the gain of the variable gain amplifier 210 is increased.
However, the automatic gain control circuit 200 in FIG. 2 still contains the following drawbacks:
1. The programmable level shifters may lead to a phase difference between differential signals, thereby affecting processing precision.
2. The programmable level shifters increase the loading along the signal path so that bandwidth of the automatic gain control circuit is reduced.
3. If the output signals have large amplitudes, programmable level shifters having a large output range and wide bandwidth are required and hence difficult to implement with low voltage circuits.
Accordingly, one object of the present invention is to provide an automatic gain control circuit and control method that not only increases the bandwidth of the automatic gain control circuit, but also permits its implementation using low voltage circuits.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, the invention provides an automatic gain control circuit capable of controlling its gain according to incoming reference signals. The gain control circuit includes a variable gain amplifier, a filtering circuit and a peak value inspection circuit. The variable gain amplifier receives differential input signals and amplifies the differential input signals by the gain to produce an amplified signal. The magnitude of the gain can be varied. The filtering circuit receives the amplified signal and produces differential output signals after filtering. The peak value inspection circuit receives the differential output signals and compares with a reference signal differentially to produce a control signal. The gain of the variable gain amplifier is controlled by the control signal.
In one embodiment of the present invention, the peak value inspection circuit includes a non-inverted phase differential comparator and an inverted phase differential comparator, an OR-gate and a charge/discharge circuit. The non-inverted phase differential comparator performs a differential comparison between the differential output signals and the reference signal to produce a non-inverted compare result signal. The inverted phase differential comparator performs a differential comparison between the inverted differential output signals and the reference signal to produce an inverted compare result signal. The OR-gate performs a logic operation between the non-inverted compare result signal and the inverted compare result signal to produce a combined compare result signal. The charge/discharge circuit utilizes the combined compare result signal to generate a control signal for controlling the gain of the circuit. The charge/discharge circuit can be a system that includes a charge-pump circuit and a capacitor, for example.
The filtering circuit according to the embodiment of the present invention further includes an amplifier circuit for generating the differential output signals. The original signal is magnified up to 14 times by the amplifier circuit and filtering is achieved by using a seven pole low-pass filter, for example.
The present invention also provides a method for controlling circuit gain according to a reference signal. The method of controlling the gain of amplified differential input signals includes the following steps. First, the differential input signals are amplified by the gain to produce an amplified signal. The magnitude of the gain can be controlled. The amplified signal is filtered to produce differential output signals. The differential output signals and the reference signal are compared to produce a control signal for controlling the gain of a variable gain amplifier.
The method of generating control signal further includes the following steps. A differential comparison between the differential output signals and the reference signal is carried out to produce a non-inverted compare result signal. A differential comparison between an inverted differential output signal and the reference signal is carried out to produce an inverted compare result signal. Thereafter, an OR operation between the non-inverted compare result signal and the inverted compare result signal is conducted to produce a combined compare result signal. According to the combined compare result signal resulting from the comparisons, the control signal for controlling the output gain is adjusted.
To provide a wider operating bandwidth, a peak value inspection circuit comprising full differential comparators is used to detect the magnitude of output signals so that the gain of the variable gain amplifier can be modified. Hence, aside from simplifying the automatic gain control circuit considerably, low voltage circuits may be used.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.